Silica container and method for producing the same

ABSTRACT

A method for producing a silica container having a rotational symmetry is provided. The method includes, forming a preliminarily molded article by feeding a powdered substrate&#39;s raw material to an inner wall of an outer frame having aspiration holes with rotating the frame, and forming a silica substrate. The preliminarily molded article is aspirated from an outer peripheral side with controlling a humidity inside the outer frame by ventilating gases present in the outer frame with charging from inside the preliminarily molded article a gas mixture comprised of an O2 gas and an inert gas and made below a prescribed dew-point temperature by dehumidification, and at the same time heated from inside the preliminarily molded article by a discharge-heat melting method with carbon electrodes, thereby making an outer peripheral part of the preliminarily molded article to a sintered body while an inner peripheral part to a fused glass body.

TECHNICAL FIELD

The present invention relates to a silica container mainly comprised ofa silica and to a method for producing it, and in particular, relates toa silica container with a low cost and a high dimensional precision andto a method for producing it.

BACKGROUND ART

A silica glass is used for a lens, a prism and a photomask of aphotolithography instrument in manufacturing of a large-scale integratedcircuit (LSI), for a TFT substrate used for a display, for a tube of aultraviolet lamp or an infrared lamp, for a window material, for areflection plate, for a cleaning container in a semiconductor industry,for a container for melting of a silicon semiconductor, and so forth.Accordingly, from the past, various methods for producing a silica glasshave been proposed.

For example, in Patent Document 1, a method (sol-gel method) in which asilicon alkoxide is hydrolyzed to a silica sol, which is then gelated toa wet gel, then to a dry gel by drying, and finally to a transparentsilica glass body by heating at high temperature is disclosed. In PatentDocument 2, a method in which a transparent silica glass is obtained bya sol-gel method from a silica sol mixture solution formed oftetramethoxy silane or tetraethoxy silane and a silica sol solutioncontaining silica fine particles is disclosed. In Patent Document 3, amethod for producing a transparent silica glass by using a siliconalkoxide and silica glass fine particles as its main raw materials,wherein a heating process at a temperature range from 200 to 1300° C. iscarried out under an oxygen gas-containing atmosphere, a further heatingprocess to 1700° C. or higher is carried out under a hydrogengas-containing atmosphere, and a heating process between the foregoingtwo heating processes is carried out under a reduced pressureatmosphere, is disclosed. In these conventional sol-gel methods,however, the produced silica glass had problems of a dimensionalprecision at the initial stage and a heat resistance under its use athigh temperature thereafter; and in addition, not only there wereproblems of release of carbon fine particles and emission of largequantities of gases such as CO and CO₂ because of high carbon content,but also the cost thereof was not so cheap.

In Patent Document 4, a method (slip casting method), wherein at leasttwo different kinds of silica glass particles, for example, silica glassfine particles and silica glass granules are mixed to obtain awater-containing suspension solution, which is then press molded andsintered at high temperature to obtain a silica-containing compositebody, is disclosed. In Patent Document 5, a method, wherein a mixedsolution (slurry) containing silica glass particles having the size of100 μm or less and silica glass granules having the size of 100 μm ormore is prepared, then the slurry is cast into a molding frame, dried,and then sintered to obtain an opaque silica glass composite material,is disclosed. In these conventional slip casting methods, however,shrinkage of a molded article in a drying process and a sinteringprocess is so significant that a thick silica glass article with a highdimensional precision could not be obtained. In addition, because ofhigh water content there were problems of the high OH concentration andof a large released of an H₂O gas during its use at high temperaturethereafter.

Accordingly, there are problems as mentioned above in the method forproducing a silica glass article from a powdered raw material.Therefore, as a method for producing a silica crucible for manufacturingof a single crystal silicon used for LSI, such production methods asthose disclosed in Patent Document 6 and Patent Document 7 are beingused still today. In these methods, after a powdered ultra-highlypurified natural quartz is fed into a rotating frame and then molded,carbon electrodes are inserted from the top and then electricallycharged to cause arc discharge thereby raising the atmospherictemperature to a temperature range for melting of the powdered quartz(estimated temperature in the range from about 1800 to about 2100° C.)so that the powdered raw quartz may be melted and sintered.

In these methods, however, there has been a problem of a high costbecause powdered raw material quartz with high purity is used. Inaddition, because various kinds of impure gases and a large quantity offine carbon particles scattered from the carbon electrodes are dissolvedor contained in a produced silica crucible, the gases are released andthen incorporated into a single crystal silicon as gaseous bubbles whenit is used as a silica crucible for growing of a single crystal silicon,thereby causing problems in production cost as well as quality of thesilicon crystal. In addition, there has been a problem of a poor thermaldistortion resistance of the silica crucible because side wall of thecrucible is distorted by softening at the time of pulling up of a singlecrystal silicon.

In Patent Document 8, a silica crucible formed of three layers of anouter layer comprised of a natural quartz glass, an intermediate layercomprised of a synthetic quartz glass containing aluminum in highconcentration, and an inner layer comprised of a high purity syntheticquartz glass, obtained from a powdered silica raw material by anarc-discharge melting method is described (it seems that the melting wascarried out under an air atmosphere). In it, prevention effect ofimpurity migration by the intermediate layer is shown. However, not onlya high cost of the three-layer structure having the structure asmentioned above but also the problems of thermal distortion resistanceand of formation of voids and pinholes contained in a single crystalsilicon have been remained unsolved.

In Patent Document 9, a technology to reduce gaseous bubbles in a wallof a melted quartz crucible by aspiration from a peripheral of a moldingframe at the time of an arc-discharge melting of a molded article of apowdered raw material silica is shown.

However, dissolved gases in a wall of a melted quartz crucible could notbe removed completely by mere aspiration of an air present in thepowdered silica. It was only possible to produce a crucible containing alarge quantity of residual gases, in particular, CO, CO₂ and H₂O.

In Patent Document 10, a silica crucible formed of three layerscontaining a crystallization facilitation agent, produced by a similararc-discharge melting method, is shown.

However, when a single crystal silicon is pulled up by using thisthree-layered crucible, there have been problems that the crucible isnot necessarily crystallized uniformly, defects such as voids andpinholes are formed in a grown single crystal silicon because of a largequantity of released gases from the crucible, and the thermal distortiontakes place at the time of using the crucible.

CITATION LIST Patent Document

-   Patent Document 1: Japanese Patent Application Laid-Open Publication    No. H07-206451-   Patent Document 2: Japanese Patent Application Laid-Open Publication    No. H07-277743-   Patent Document 3: Japanese Patent Application Laid-Open Publication    No. H07-277744-   Patent Document 4: Japanese Patent Application Laid-Open Publication    No. 2002-362932-   Patent Document 5: Japanese Patent Application Laid-Open Publication    No. 2004-131380-   Patent Document 6: Japanese Examined Patent Application Publication    No. H04-22861-   Patent Document 7: Japanese Examined Patent Application Publication    No. H07-29871-   Patent Document 8: Japanese Patent Application Laid-Open Publication    No. H09-255476-   Patent Document 9: Japanese Patent Application Laid-Open Publication    No. H10-25184-   Patent Document 10: Japanese Patent Application Laid-Open    Publication No. H11-171684

SUMMARY OF THE INVENTION Technical Problem to be Solved by the Invention

The present invention was made in view of the problems as mentionedabove, and has an object to provide a method for producing a silicacontainer comprised of a silica as the main component therein, having ahigh dimensional precision, and containing carbon and an OH group withextremely low amounts in almost entire region of the silica container,by using powders containing a silica as the main raw material with a lowproduction cost, and an object to provide a silica container such as theone as mentioned above.

Solution to Problem

The present invention was made in order to solve the problems asmentioned above and provides a method for producing a silica containercomprised of at least a silica as its main component and arranged with asilica substrate having a rotational symmetry, wherein the methodcomprises:

a step of preparing a powdered substrate's raw material comprised ofsilica particles for forming the silica substrate,

a step of forming a preliminarily molded silica substrate, wherein thepowdered substrate's raw material is fed to an inner wall of an outerframe having a rotational symmetry and aspiration holes arrangedsplittingly in the inner wall while rotating the outer frame therebypreliminarily molding the powdered substrate's raw material to anintended shape in accordance with the inner wall of the outer frame, and

a step of forming the silica substrate, wherein the preliminarily moldedsilica substrate is degassed by aspiration from an outer peripheral sidethrough the aspiration holes formed in the outer frame with controllinga humidity inside the outer frame by ventilating gases present in theouter frame with charging from inside the preliminarily molded silicasubstrate a gas mixture comprised of an O₂ gas and an inert gas and madebelow a prescribed dew-point temperature by dehumidification, and at thesame time heated from inside the preliminarily molded silica substrateby a discharge-heat melting method with carbon electrodes, therebymaking an outer peripheral part of the preliminarily molded silicasubstrate to a sintered body while an inner peripheral part of thepreliminarily molded silica substrate to a fused glass body.

If the method for producing a silica container includes the steps asmentioned above, fine carbon particles scattered from the carbonelectrodes are gasified by the oxidation treatment thereby enabling toreduce carbon (C) contained in the producing silica substrate to anextremely low amount; thus a harmful effect by carbon, carbon monoxide(CO), and carbon dioxide (CO₂) to a material accommodated in theproduced silica container can be suppressed, and at the same time thethermal distortion resistance under the use condition of the producedsilica container at high temperature can be improved because an OH groupcontained in the silica substrate can be lowered to an extremely lowconcentration so that viscosity of the silica substrate can be madehigher.

In addition, the present invention can be executed without adding aspecial equipment and a process step to a conventional method, so that asilica container containing extremely low amounts of carbon and an OHgroup can be produced with a high dimensional precision, a highproductivity, and a low cost.

The method for producing a silica container of the present invention canfurther include, after the step of forming the silica substrate by thedischarge-heat melting method, a step of forming an inner layer formedof a transparent silica glass on an inner surface of the silicasubstrate, by spreading from inside the silica substrate a powderedinner-layer's raw material, comprised of silica particles and having ahigher silica purity than the powdered substrate's raw material, withheating from inside the silica substrate by a discharge-heat meltingmethod.

Accordingly, if the method for producing a silica container is made toinclude further a step of forming an inner layer formed of a transparentsilica glass on an inner surface of the obtained silica substrate,impurity contamination to a material accommodated in the produced silicacontainer can be reduced more effectively.

In this case, the powdered inner-layer's raw material can be made to theone that releases H₂ the amount of which is in the range from 1×10¹⁶ to1×10¹⁹ molecules/g at 1000° C. under vacuum.

If the powdered inner-layer's raw material is doped with H₂, the releaseamount of which at 1000° C. under vacuum is in the range from 1×10¹⁶ to1×10¹⁹ molecules/g, the powdered inner-layer's raw material contains anH₂ molecule releasing this amount of H₂ molecules, so that, because ofthe presence of this H₂ molecules, the amount of gaseous bubbles in thesilica glass layer that constitutes the inner layer can be reduced tomake the silica glass layer a more perfect transparent layer.

In addition, in the powdered inner-layer's raw material, it ispreferable that each element concentration of Li, Na, and K be made 60or less ppb by weight, and each element concentration of Ti, V, Cr, Fe,Co, Ni, Cu, Zn, Mo, and W be made 30 or less ppb by weight.

If the metals are contained in the powdered inner-layer's raw materialwith the concentrations as mentioned above, impurity contamination to amaterial accommodated in the produced silica container can be reducedmore effectively.

In addition, in the method for producing a silica container of thepresent invention, it is preferable that the ratio of an O₂ gascontained in the gas mixture be in the range from 1 to 40% by volume.

If the ratio of an O₂ gas contained in the gas mixture is made to therange as mentioned above, carbon fine particles scattered can be removedmore effectively.

In addition, it is preferable that the dehumidification be done suchthat a dew-point temperature of the gas mixture is 10° C. or lower.

If the dehumidification of the gas mixture is executed such that adew-point temperature of the gas mixture may become 10° C. or lower asmentioned above, amount of an OH group contained in the silica containerto be produced can be reduced effectively.

In addition, the powdered substrate's raw material can be made tocontain aluminum in the concentration rage from 10 to 500 ppm by weight.

If the powdered substrate's raw material is made to contain aluminum inthe concentration rage from 10 to 500 ppm by weight as mentioned above,diffusion of metal impurities in the silica substrate can be suppressed,so that impurity contamination to a material accommodated in the silicacontainer can be reduced.

Further, the present invention provides a silica container comprised ofat least a silica as its main component and arranged with a silicasubstrate having a rotational symmetry, wherein the silica substratecontains carbon in the concentration of 30 or less ppm by weight and anOH group in the concentration of 30 or less ppm by weight and has awhite and opaque layer part containing gaseous bubbles in its outerperipheral part and a colorless and transparent layer part comprised ofa silica glass not substantially containing gaseous bubbles in its innerperipheral part.

If the silica container is the one as mentioned above, amount of carbon(C) contained in the silica substrate is extremely small so that aharmful effect by carbon, carbon monoxide, and carbon dioxide to amaterial accommodated in the silica container can be suppressed; and atthe same time, amount of an OH group contained in the silica substrateis extremely small thereby enabling to make viscosity of the silicasubstrate higher, so that the thermal distortion resistance of thesilica container under the use condition at high temperature can beimproved.

In this case, it is preferable that, when the colorless and transparentlayer part of the silica substrate is heated at 1000° C. under vacuum,amounts of released gas molecules be 2×10¹⁷ or less molecules/g for a COmolecule and 1×10¹⁷ or less molecules/g for a CO₂ molecule.

If the released amounts of a CO molecule and a CO₂ molecule, when thecolorless and transparent layer part of the silica substrate is heatedat 1000° C. under vacuum, are the value as mentioned above, a harmfuleffect by a CO gas molecule and a CO₂ gas molecule to a materialaccommodated in the silica container can be suppressed more effectively.

In addition, it is preferable that, when the colorless and transparentlayer part of the silica substrate is heated at 1000° C. under vacuum,amount of a released H₂O molecule be 3×10¹⁷ or less molecules/g.

If the released amount of an H₂O molecule, when the colorless andtransparent layer part of the silica substrate is heated at 1000° C.under vacuum, is 3×10¹⁷ or less molecules/g, a harmful effect by an H₂Ogas molecule to a material accommodated in the silica container can besuppressed.

In addition, it is preferable that viscosity of the colorless andtransparent layer part of the silica substrate at 1400° C. be 10^(10.4)Pa·s or higher.

If viscosity of the colorless and transparent layer part of the silicasubstrate at 1400° C. is 10^(10.4) Pa·s or higher as mentioned above,the silica container can be made to have a high thermal distortionresistance; and thus the silica container can be used with suppresseddistortion even at such high temperature as, for example, 1400° C. orhigher.

In addition, it is preferable that the silica substrate contain aluminumin the concentration range from 10 to 500 ppm by weight.

If the silica substrate contains aluminum in the concentration rangefrom 10 to 500 ppm by weight as mentioned above, diffusion of metalimpurities in the silica substrate can be suppressed so that impuritycontamination to a material accommodated in the silica container can bereduced.

In addition, the silica container of the present invention can bearranged with, on an inner surface of the silica substrate, an innerlayer formed of a transparent silica glass having a higher silica puritythan the silica substrate.

If any of the silica containers as mentioned above is arranged with, onan inner surface of the silica substrate, an inner layer formed of atransparent silica glass having a higher silica purity than the silicasubstrate, impurity contamination to a material accommodated in thesilica container can be reduced more effectively.

In this case, it is preferable that the inner layer contain carbon withthe concentration of 30 or less ppm by weight, an OH group with theconcentration of 30 or less ppm by weight, an element of Li, Na, and Kwith each element concentration of 60 or less ppb by weight, and anelement of Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo, and W with each elementconcentration of 30 or less ppb by weight.

If concentrations of carbon, an OH group, and each metal contained inthe inner layer are in the values as mentioned above, impuritycontamination to a material accommodated in the produced silicacontainer can be reduced more effectively.

Advantageous Effects of the Invention

As described above, according to the method for producing a silicacontainer of the present invention, amount of carbon (C) contained inthe silica substrate to be produced can be made extremely small so thata harmful effect by carbon, carbon monoxide, and carbon dioxide to amaterial accommodated in the produced silica container can besuppressed; and at the same time the OH concentration contained in thesilica substrate can be made extremely low so that viscosity of thesilica substrate can be made high thereby enabling to improve thethermal distortion resistance under the use condition of the producedsilica container at high temperature. In addition, the present inventioncan be executed without adding a special equipment and a process step toa conventional method so that the silica container containing extremelylow amounts of carbon and an OH group can be produced in a highdimensional precision, a high productivity, and a low cost.

Further, according to the silica container of the present invention, aharmful effect by carbon, carbon monoxide, and carbon dioxide to amaterial accommodated in the silica container can be suppressed; and inaddition, the thermal distortion resistance under the use condition ofthe silica container at high temperature can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross section view showing one example of thesilica container of the present invention.

FIG. 2 is a schematic cross section view showing another example of thesilica container of the present invention.

FIG. 3 is a flow chart showing outline of one example of the method forproducing a silica container of the present invention.

FIG. 4 is a flow chart showing outline of another example of the methodfor producing a silica container of the present invention.

FIG. 5 is a schematic cross section view showing one example of theouter frame usable in the method for producing a silica container of thepresent invention.

FIG. 6 is a schematic cross section view schematically showing oneexample of the step of forming the preliminarily molded silica substratein the method for producing a silica container of the present invention.

FIG. 7 is a schematic cross section view schematically showing a part ofone example of the step of forming the silica substrate (beforedischarge-heat melting) in the method for producing a silica containerof the present invention.

FIG. 8 is a schematic cross section view schematically showing a part ofone example of the step of forming the silica substrate (duringdischarge-heat melting) in the method for producing a silica containerof the present invention.

FIG. 9 is a schematic cross section view schematically showing oneexample of the step of forming the inner layer in the method forproducing a silica container of the present invention.

DESCRIPTION OF EMBODIMENTS

As mentioned above, in a conventional method for producing a silicacontainer, there have been problems in a dimensional precision and in acost.

In addition, a silica container produced by a conventional method forproducing a silica container had a problem such as, for example, aneffect of gases to a material accommodated therein caused byincorporation of gaseous bubbles into a silicon single crystal in asilica crucible for growing of a silicon single crystal.

The inventors of the present invention carried out investigation in viewof the problems as mentioned above and found the following problems tobe solved.

Firstly, a silica container such as a crucible and a boat for melting ofa metal silicon and for production of a silicon crystal requires thermaluniformity inside the container under atmosphere of high temperatureheating. Because of this, the first problem is to make the silicacontainer at least a multi-layer structure, wherein an outside part ofthe container is made to a white and opaque porous silica glass while aninside part of the container is made to a colorless and transparentsilica glass containing substantially little gaseous bubbles.

The second problem is to make the silica container contain reducedamounts of carbon fine particles and of dissolved gases such as an H₂Ogas (a water molecule), a CO gas (a carbon monoxide molecule), and a CO₂gas (a carbon dioxide molecule).

This is to suppress a harmful effect to a material accommodated in thesilica container by carbon fine particles and gaseous molecules such anH₂O gas, a CO gas, and a CO₂ gas contained in the silica container.

For example, in the case of a silica container used for pulling up of asingle crystal silicon, if carbon fine particles are contained in thesilica container, the carbon fine particles contained in the silicacontainer are released and dissolved into a silicon melt at the timewhen silicon crystal is produced; and therefore the carbon fineparticles occasionally work as a crystal nucleating agent to inhibitgrowth of the single crystal silicon. In addition, if gaseous moleculessuch as an H₂O gas, a CO gas, and a CO₂ gas are incorporated into thesilica container, these gases are released into a silicon melt at thetime when silicon crystal is produced and are then incorporated into agrowing single crystal silicon as gaseous bubbles. The gaseous bubblesincorporated therein form a void and a pinhole when the single crystalsilicon is made to a wafer, thereby leading to remarkable decrease inproduction yield.

As mentioned above, in the present invention, it was necessary tosimultaneously solve these two technical problems with a lower cost ascompare with a silica container such as a crucible for pulling up of ahigh purity single crystal silicon by a conventional method.Accordingly, the third problem to be solved was to obtain a low costproduction method by using a cheap silica raw material not requiring atreatment for ultrahigh purification.

Hereinafter, the present invention will be explained in detail withreferring to the figures, but the present invention is not limited tothem. Particularly in the following, mainly a silica container (asolar-grade crucible) applicable as a container for melting of a metalsilicon used as a material for a solar cell (a solar photovoltaic powergeneration, or a solar power generation) as well as a production methodthereof will be mainly explained as one suitable example of applicationof the present invention; but the present invention is not limited tothis and can be applied widely to a general silica container comprisedof a silica as the main component and having a rotational symmetry.

In FIG. 1, a schematic cross section view of one example of the silicacontainer of the present invention is shown.

The silica container 71 of the present invention has a rotationalsymmetry, and its basic structure is formed of the silica substrate 51.The silica substrate 51 contains carbon in the concentration of 30 orless ppm by weight and an OH group in the concentration of 30 or lessppm by weight.

The silica substrate 51 has a white and opaque layer part 51 acontaining gaseous bubbles in its outer peripheral part and a colorlessand transparent layer part 51 b comprised of a silica glass notsubstantially containing gaseous bubbles in its inner peripheral part.

Meanwhile, as far as the silica container of the present invention hasat least the silica substrate 51, the silica container may furthercontain a layer other than the silica substrate.

In FIG. 2, the silica container 71′ arranged with the inner layer 56formed of a transparent silica glass on an inner surface of the silicasubstrate 51 is shown as another example of the silica container of thepresent invention. It is preferable that a silica purity of the innerlayer 56 be higher than that of the silica substrate 51.

Hereinafter, the silica substrate 51 that constitutes the silicacontainer of the present invention will be specifically explained.

Firstly, the amounts of carbon (C) and an OH group contained in thesilica substrate 51 are made in the concentration of 30 or less ppm byweight for carbon and in the concentration of 30 or less ppm by weightfor an OH group. It is preferable that the concentration of carbon be 10or less ppm by weight and the concentration of an OH group be 10 or lessppm by weight.

If the amounts of carbon C and an OH group contained in the silicasubstrate 51 are the values as mentioned above, the amount of carbon (C)contained in the silica substrate 51 is extremely small, so that aharmful effect by carbon, carbon monoxide, and carbon dioxide to amaterial accommodated in the silica container 71 or 71′ can besuppressed; and at the same time the amount of an OH group contained inthe silica substrate 51 is extremely small so that viscosity of thesilica substrate 51 can be made higher, and thus the thermal distortionresistance under the use condition of the silica container 71 or 71′ athigh temperature can be improved.

The silica substrate 51 has a white and opaque layer part 51 acontaining gaseous bubbles in its outer peripheral part and a colorlessand transparent layer part 51 b comprised of a silica glass notsubstantially containing gaseous bubbles in its inner peripheral part,as mentioned above. Because the silica substrate 51 has the white andopaque layer part 51 a and the colorless and transparent layer part 51b, as mentioned above, thermal uniformity inside the silica containerunder heating condition can be improved.

Bulk density of the white and opaque layer part 51 a can be made, forexample, in the range from 1.90 to 2.20 g/cm³, and bulk density of thecolorless and transparent layer part 51 b can be made typically about2.20 g/cm³; but the present invention is not particularly limited tothese values.

The silica containers 71 and 71′ are used at high temperature underreduced pressure in many cases so that the amounts of released gasesfrom the silica containers 71 and 71′ need to be made small under suchconditions. Accordingly, it is preferable that the amount of gasmolecules released from the colorless and transparent layer part 51 bwhen heated at 1000° C. under vacuum be 2×10¹⁷ or less molecules/g for aCO molecule and 1×10¹⁷ or less molecules/g for a CO₂ molecule.

In addition, when the colorless and transparent layer part 51 b of thesilica substrate is heated at 1000° C. under vacuum, the amount of areleased H₂O molecule therefrom is preferably 3×10¹⁷ or lessmolecules/g, or more preferably 1×10¹⁷ or less molecules/g.

As to an H₂ gas, it is preferable that the amount of an H₂ gas releasedfrom the colorless and transparent layer part 51 b of the silicasubstrate when heated at 1000° C. under vacuum be 5×10¹⁶ or lessmolecules/g.

If the amount of each gas molecules dissolved in the silica substrate 51is suppressed as mentioned above, a harmful effect by each gas moleculesto a material accommodated in the silica container can be reduced. Forexample, when the silica container 71 of the present invention is usedfor pulling up of a single crystal silicon, if the foregoing gases arereleased, the gases are incorporated into the silicon crystal to causestructural defects by gaseous bubbles such as so-called a void and apinhole in the crystal; but according to the present invention, thisharmful effect can be reduced.

In addition, by adding aluminum into the silica substrate 51 preferablyin the concentration range from 10 to 500 ppm by weight or morepreferably in the concentration range from 50 to 500 ppm by weight,adsorption and immobilization of impure metal elements become possible.

Details of a mechanism for aluminum to prevent migration and diffusionof impure metal elements in the silica glass from occurring is notknown; but it is assumed that a positive ion (cation) of impure metalelements balances in its electric charge with a silica glass network bydisplacing Si with Al, resulting in adsorption as well as prevention ofdiffusion from occurring.

An additional effect of adding aluminum into the silica substrate 51 isto increase viscosity of a silica glass at high temperature therebyenabling to improve the thermal distortion resistance of the silicasubstrate 51 at high temperature; accordingly, the thermal distortionresistance of the silica containers 71 and 71′ can be improved.

On the other hand, when concentration of an OH group is made 30 or lessppm by weight or preferably 10 or less ppm by weight as mentioned above,decrease of the viscosity of a silica glass at high temperature can besuppressed; and thus effects of adsorption and immobilization of theforegoing impure metal elements can be obtained.

In the present invention, viscosity of the silica substrate 51 can beincreased by such methods as making concentration of an OH group in thesilica substrate 51 to the foregoing value and adding aluminum; withthis, the thermal distortion resistance of the silica containers 71 and71′ can be improved. Specifically, viscosity of the colorless andtransparent layer part 51 b of the silica substrate 51 at 1400° C. canbe made to 10^(10.4) Pa·s or higher, or more preferably 10^(10.5) Pa·sor higher as well.

Meanwhile, viscosity of the colorless and transparent layer part 51 b at1400° C. can be calculated by a beam bending method and the like.

Then, the inner layer 56 of the silica container 71′ shown in FIG. 2will be explained.

The inner layer 56 is formed on an inner wall of the silica substrate 51and formed of a transparent silica glass having a higher silica puritythan the silica substrate.

If the inner layer 56 as mentioned above is formed, for example, silicapurity of the silica substrate 51 can be made relatively low, in therange from 99.9 to 99.999% by weight. If the silica container 71′arranged with the inner layer 56 is used, impurity contamination to amaterial accommodated therein can be fully avoided even if the low-costsilica container having the silica substrate 51 with the silica purityas mentioned above is used.

Further, the concentration of carbon (C) contained in the inner layer 56is preferably 30 or less ppm by weight and more preferably 10 or lessppm by weight, for the same reason as the concentration of carboncontained in the silica substrate 51.

In addition, it is preferable that the concentration of an OH groupcontained in the inner layer 56 be 30 or less ppm by weight and morepreferably 10 or less ppm by weight. An OH group contained in the innerlayer 56 has an effect to decrease the diffusion rate of impure metalelements, but on the contrary has an effect to decrease an etchingresistance; and thus, the concentration is limited in an appropriaterange.

It is preferable that the inner layer 56 contain each element of Li, Na,and K in the concentration of 60 or less ppb by weight, and each elementof Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo, and W in the concentration of 30or less ppb by weight. More preferably, concentration of each element ofLi, Na, and K is 20 or less ppb by weight and concentration of eachelement of Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo, and W is 10 or less ppb byweight. With this, a harmful effect by these impure elements to amaterial to be treated can be reduced. Especially, in the case that amaterial to be treated is a silicon crystal for solar use, decrease inthe incident photon-to-current conversion efficiency can be prevented sothat quality of it can be made high.

Hereinafter, a method for producing the silica containers 71 and 71′ asmentioned above will be explained specifically. Especially, a method forproducing a silica container producible in a low cost (solar gradecrucible) usable as a container for melting of a metal silicon (Si) andpulling up of a single crystal, used as a material for solarphotovoltaic power generation device and the like, will be explained asthe example.

A schematic diagram of the method for producing the silica container 71of the present invention is shown in FIG. 3.

Firstly, a powdered substrate's raw material 11 comprised of silicaparticles is prepared as shown in FIG. 3 (1) (Step 1).

The powdered substrate's raw material 11 will become a main compositionmaterial of the silica substrate 51 in the silica containers 71 and 71′of the present invention (refer to FIG. 1 and FIG. 2).

The powdered substrate's raw material can be obtained by crushing massof silica and granulating the powders thereby obtained, for example, bythe method as described below, though not limited to it.

Firstly, mass of natural silica (a naturally produced berg crystal, aquartz, a silica, a silica stone, an opal stone, and the like) withdiameter in the range from about 5 to about 50 mm is heated in thetemperature ranging from 600 to 1000° C. for about 1 to about 10 hoursunder an air atmosphere. Then, the mass of natural silica thus treatedis poured into water to be cooled down quickly, separated, and thendried. With these treatments, subsequent crushing by a crusher or thelike and granulation of the obtained powders can be executed easily; butcrushing treatment may be executed without executing the foregoingheating and quick cooling treatments.

Then, mass of the natural silica thus treated is crushed by a crusher orthe like, and then granulated to the particle diameter controlledpreferably in the range from 10 to 1000 μm, or more preferably in therange from 50 to 500 μm, to obtain a powdered natural silica.

Thereafter, the powdered natural silica thus obtained is heated in thetemperature range from 700 to 1100° C. for about 1 hour to about 100hours in a rotary kiln formed of a silica glass tube having aninclination angle, inside of which is made to an atmosphere containing ahydrogen chloride gas (HCl) or a chlorine gas (Cl₂) forhigh-purification treatment. However, for the use not requiring a highpurity, this treatment for high purification may be omitted to proceedto the subsequent steps.

The powdered substrate's raw material 11 obtained after the foregoingsteps is a crystalline silica; but depending on the use purpose of thesilica container, an amorphous silica glass scrap may be alternativelyused as the powdered substrate's raw material 11.

Diameter of the powdered substrate's raw material 11 is preferably inthe range from 10 to 1000 μm, or more preferably in the range from 50 to500 μm, as mentioned above.

Silica purity of the powdered substrate's raw material 11 is preferably99.99% or higher by weight, or more preferably 99.999% or higher byweight. According to the method for producing a silica container of thepresent invention, even if silica purity of the powdered substrate's rawmaterial 11 is made relatively low, such a 99.999% or lower by weight,impurity contamination to a material accommodated in the produced silicacontainer can be fully avoided. Accordingly, the silica container can beproduced with a lower cost as compared with the conventional methods.

When the silica container 71′ arranged with the inner layer 56 as shownin FIG. 2 is produced (this method will be described later), especiallya silica purity of the powdered substrate's raw material 11 can belowered; for example, a purity of 99.9% or higher by weight may beallowed.

Further, the powdered substrate's raw material 11 may be made to containaluminum preferably in the range from 10 to 500 ppm by weight.

Aluminum is added, for example, as an aqueous or alcoholic solution ofan aluminum salt of nitrate, acetate, carbonate, chloride, and the like;and into a solution thereof a powdered silica is added and soaked, andthen dried.

In the present invention, it is preferable that, in a step of preparingthe powdered substrate's raw material 11, concentration of an OH groupcontained in the powdered substrate's raw material 11 be as low aspossible.

The amount of an OH group contained in the powdered substrate's rawmaterial 11 may be as it is contained in the natural silica stone fromthe beginning; or alternatively the amount of water to be incorporatedin the intermediate step can be controlled by gas atmosphere, treatmenttemperature, and time, in a step of drying followed thereafter; but, atany rate, it is preferable that content of an OH group be as low aspossible in the present invention.

After the powdered substrate's raw material 11 is prepared as mentionedabove, the powdered substrate's raw material is fed into an outer framehaving a rotational symmetry for molding of the powdered substrate's rawmaterial 11, as shown in FIG. 3 (2) (Step 2).

In FIG. 5, a schematic cross section view of the outer frame forpreliminary molding of the powdered substrate's raw material 11 isshown. The outer frame 101 used in the present invention is formed of,for example, a material member such as graphite and has a rotationalsymmetry. In the inner wall 102 of the outer frame 101, the aspirationholes 103 are arranged splittingly. The aspiration holes 103 areconnected to the aspiration path 104. The rotation axis 106 to rotatethe outer frame 101 is also arranged with the aspiration path 105,through which aspiration can be done.

The powdered substrate's raw material 11 is fed into the inner wall 102of the outer frame 101 to preliminarily mold the powdered substrate'sraw material 11 to a prescribed shape in accordance with the inner wall102 of the outer frame 101 thereby giving the preliminarily moldedsilica substrate 41 (refer to FIG. 6).

Specifically, the powdered substrate's raw material 11 is fed graduallyinto the inner wall 102 of the outer frame 101 from a hopper for thepowdered raw material (not shown) while rotating the outer frame 101thereby molding to a shape of the container by utilizing the centrifugalforce. Alternatively, thickness of the preliminarily molded silicasubstrate 41 may be controlled to the prescribed value by contacting aplate-like inner frame (not shown) to the rotating powders from inside.

A feeding method of the powdered substrate's raw material 11 into theouter frame 101 is not particularly limited; for example, a hopperequipped with an agitation screw and with a measuring feeder may beused. In this case, the powdered substrate's raw material 11 filled inthe hopper is fed with agitating by the agitation screw whilecontrolling the feeding amount by the measuring feeder.

Then, as shown in FIG. 3 (3), the silica substrate 51 is formed by thedischarge-heat melting method under aspiration (Step 3).

Specifically, as shown in FIG. 7 and FIG. 8, the preliminarily moldedsilica substrate 41 is degassed by aspiration from the outer peripheralside of the preliminarily molded silica substrate 41 through theaspiration holes 103 formed in the outer frame 101, while simultaneouslyheating from inside of the preliminarily molded silica substrate by thedischarge-heat melting method. With this, the silica substrate 51 havinga sintered body in the outer peripheral part of the preliminarily moldedsilica substrate 41 and a fused glass body in the inner part of thepreliminarily molded silica substrate 41 is formed. At this time, theforegoing melting and sintering are executed by the discharge-heatingmethod with controlling a humidity inside the outer frame 101 byventilating the gases present in the outer frame 101 with charging frominside the preliminarily molded silica substrate 41 a gas mixturecomprised of an O₂ gas and an inert gas and made below a prescribeddew-point temperature by dehumidification.

The equipment for forming the silica substrate 51 is comprised of, inaddition to the rotatable outer frame 101 having a revolution axissymmetry as mentioned above, the rotation motor (not shown), the carbonelectrodes 212 which are the heat source of the discharge-heat melting(sometimes called arc melting or arc discharge melting), the electricwirings 212 a, the high voltage electricity source unit 211, the cap213, and so forth. In addition, structural components to control anatmospheric gas to be charged from inside the preliminarily moldedsilica substrate, for example, the O₂ gas-supplying cylinder 411, theinert gas-supplying cylinder 412, the gas mixture-supplying pipe 420,the dehumidifying equipment 430, the dew-point-temperature-measuringthermometer 440, and so forth are arranged.

Meanwhile, the equipment can also be used successively for formation ofthe inner layer 56 on the inner surface of the silica substrate 51, aswill be described later.

Procedures for melting and sintering of the preliminarily molded silicasubstrate 41 are as following; before start of the electricity chargebetween the carbon electrodes 212, charge of a gas mixture, comprised ofan O₂ gas and an inert gas and made below a prescribed dew-pointtemperature by dehumidification, from inside the preliminarily moldedsilica substrate 41 is started. Specifically, as shown in FIG. 7, an O₂gas in the O₂ gas-supplying cylinder 411 and an inert gas (for example,nitrogen (N₂), argon (Ar), and helium (He)) in the inert gas-supplyingcylinder 412 are mixed and charged from inside the preliminarily moldedsilica substrate 41 through the gas mixture-supplying pipe 420.Meanwhile, white hollow arrows shown by the code number 510 show theflow direction of the gas mixture.

Dehumidification can be done by an appropriate dehumidifying equipmentand the like, and measurement of the dew point temperature can be donewith an appropriate dew-point-temperature-measuring thermometer and thelike. In FIG. 7, an embodiment in which the dehumidifying equipment 430and the dew-point-temperature-measuring thermometer 440 are integratedto the gas mixture-supplying pipe 420 is shown, but the embodiment isnot limited to this; any embodiment enabling to make the dew pointtemperature of the gas mixture below a prescribed value can be used.

At this time, a gas in the outer frame 101 is ventilated simultaneously,as mentioned above. The ventilation can be done by escaping theatmospheric gas in the outer frame 101 to outside, for example, througha space in the cap 213. Meanwhile, white hollow arrows shown by the codenumber 520 show the flow direction of the atmospheric gas withventilation.

Meanwhile, in humidity control of the gas mixture, it is preferable thatthe dehumidification be done such that a dew-point temperature of thegas mixture may become 10° C. or lower. In humidity control of the gasmixture, the dehumidification is done such that more preferably adew-point temperature of the gas mixture may become 5° C. or lower, orstill more preferably below −15° C.

As mentioned above, the humidity in the outer frame 101 is controlled bycharging the gas mixture whose dew-point temperature is made below theprescribed value while ventilating the gas inside the outer frame 101.

Then, under the continuing condition of controlling the humidity in theouter frame 101 by charging the gas mixture whose dew-point temperatureis made below the prescribed value while ventilating the gases insidethe outer frame 101 as described above, a vacuum pump for degassing (notshown) is started thereby aspirating the preliminarily molded silicasubstrate 41 from its outer side through the aspiration holes 103 andthe aspiration paths 104 and 105 and at the same time charging ofelectricity between the carbon electrodes 212 is started while rotatingthe outer frame 101 having the preliminarily molded silica substrate 41at a certain constant rate.

When the arc discharge between the carbon electrodes 212 is started(shown by the numeral code 220), temperature of the inner surface partof the preliminarily molded silica substrate 41 reaches melting regionof the powdered silica (estimated temperature in the range from about1800 to about 2000° C.) thereby starting to melt from the most surfacelayer part. When the most surface layer part is melted, degree of vacuumby aspiration with the vacuum pump for degassing increases (pressure isdropped rapidly), the change to a fused silica glass layer progressesfrom inside to outside while dissolved gases contained in the powderedsubstrate's raw material 11 are being degassed.

Heating by the electric charge and aspiration by the vacuum pump arecontinued until about half of inside the entire silica substratethickness is fused thereby forming the transparent to semitransparentlayer part 51 b (transparent layer part), while about half of outsideremained becomes the sintered white and opaque silica 51 a (opaque layerpart). Degree of vacuum is made preferably 10⁴ Pa or lower, or morepreferably 10³ Pa or lower.

The silica container 71 of the present invention may be made only withthe silica substrate 51 formed by the processes up to this stage.

The atmospheric gas at the time when inside the silica substrate 51 isfused and sintered by the discharge-heating is under the condition ofcontrolled humidity in the outer frame 101 by charging the gas mixturehaving its dew-point temperature made below the prescribed value andcomprised of an O₂ gas and an inert gas while ventilating the gasesinside the outer frame 101.

Because of this, the scattered carbons from the carbon electrodes 212are gasified by oxidation with an O₂ gas; and thus the amount of carbon(C) contained in the silica substrate 51 can be made extremely small. Asa result, a harmful effect to a material accommodated in the producedsilica container 71 by carbon, carbon monoxide (CO), and carbon dioxide(CO₂) can be decreased. In addition, because the humidity is controlledto be kept at a low value so that the concentration of an OH groupcontained in the silica substrate 51 can be made extremely low, evenbelow the desired value, thereby enabling to increase viscosity of thesilica substrate 51; and thus the thermal distortion resistance underthe use condition of the produced silica container 71 at hightemperature can be improved. Further, an H₂O gas molecule dissolved inthe silica substrate 51 can be reduced.

Meanwhile, if the ratio of an O₂ gas contained in the foregoing gasmixture is made in the range from 1 to 40% by volume, the scatteredcarbon fine particles can be removed more effectively; and thus thisrange is preferable.

The silica container 71 of the present invention can be made only withthe silica substrate 51 formed by the processes up to this stage, but asappropriate, as shown in FIG. 2, the inner layer 56 may be formed on theinner surface of the silica substrate 51, whereby the silica container71′ arranged with the silica substrate 51 and the inner layer 56 can bemade to the silica container of the present invention.

The method for producing the silica container 71′ arranged with theinner layer 56 as shown in FIG. 2 will be explained with referring toFIG. 4.

Firstly, the steps up to form the silica substrate 51 are executed in asimilar manner to those of Steps 1 to 3 as shown in the foregoing FIGS.3 (1) to (3) (refer to FIGS. 4 (1) to (3)).

Then, as shown in FIG. 4 (4), the silica substrate 51 is heated from itsinside by the discharge-heat melting method while the powderedinner-layer's raw material 12 comprised of silica particles and having ahigher silica purity than the powdered substrate's raw material 11 isspread from inside the silica substrate 51, thereby forming the innerlayer 56 on the inner surface of the silica substrate 51 (Step 4).

Alternatively, by repeating this Step 4, the inner layer 56 may beformed of a plurality of transparent silica glass layers havingdifferent purity and containing additives.

A basic method for forming the inner layer 56 is according to, forexample, the contents described in Patent Document 6 and Patent Document7.

Firstly, the powdered inner-layer's raw material 12 is prepared.

The material for the powdered inner-layer's raw material 12 includes apowdered natural quartz, which is purified to ultra high, a powderednatural berg crystal, a powdered synthetic cristobalite, and a powderedsynthetic silica glass. A powdered crystalline silica is preferable forthe purpose to reduce gaseous bubbles in the inner layer 56, whilesynthetic powders are preferable for the purpose of forming atransparent layer with a high purity. Particle diameter is preferably inthe range from 10 to 1000 μm, or more preferably in the range from 100to 500 μm. Purity is preferably 99.9999% or higher by weight as thesilica component (SiO₂); while it is preferable that the concentrationof each element of Li, Na, and K be 60 or less ppb by weight, and theconcentration of each element of Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo and Wbe 30 or less ppb by weight. It is more preferable that theconcentration of each element of Li, Na, and K be 20 or less ppb byweight, and the concentration of each element of Ti, V, Cr, Fe, Co, Ni,Cu, Zn, Mo and W be 10 or less ppb by weight.

The powdered inner-layer's raw material 12 may be added (doped) with anH₂ gas.

Owing to an H₂ molecule contained in the powdered inner-layer's rawmaterial 12, the amount of gaseous bubbles in the transparent silicaglass layer that constitutes the inner layer 56 can be reduced so that amore perfect transparent layer may be formed. A mechanism of reducinggaseous bubbles in the fused transparent silica glass layer by an H₂ gascontained therein is not perfectly clear; but it may be assumed that anoxygen gas (O₂) (it has a large molecular diameter) contained in thepowdered inner-layer's raw material 12 reacts with hydrogen to formwater (H₂O), which is then released outside. In addition, H₂ itself issmall in its molecular diameter and has a large diffusion coefficient;and thus even if H₂ is remained in the powdered inner-layer's rawmaterial, it will not become a cause for generation of gaseous bubblesin the inner layer 56 in the subsequent steps.

Specifically, the amount of added H₂ is such that the amount of the H₂released from the powdered inner-layer's raw material 12 when heated at1000° C. under vacuum may be preferably in the range from 1×10¹⁶ to1×10¹⁹ molecules/g, or more preferably in the range from 5×10¹⁶ to5×10¹⁸ molecules/g. Meanwhile, if the particle diameter of the powderedinner-layer's raw material 12 is in the range from about 10 to about1000 μm as mentioned above, almost all of H₂ is released at 1000° C.under vacuum; and thus, the amount of released H₂ at 1000° C. undervacuum is nearly equal to the dissolved amount of an H₂ moleculecontained in the powdered inner-layer's raw material 12.

An H₂ gas can be contained into the powdered inner-layer's raw material12, for example, by heating the powdered inner-layer's raw material 12in an air-tight electric furnace made of a stainless steel jacket underan atmospheric gas comprised of 100% by volume of a hydrogen gas in thepressure range from 1 to 10 kgf/cm² (namely, in the range from about 10⁵to about 10⁶ Pa, or about 1 to about 10 atmospheres), in the temperaturerange from 200 to 800° C. or preferably in the range from 400 to 600° C.and in the time range from about 1 to about 10 hours.

The method for forming the inner layer 56 will be explained withreferring to FIG. 9.

The equipment for forming the inner layer 56 on the inner surface of thesilica substrate 51 is comprised of, in similar to those of the previousstep, the rotatable outer frame 101 having a revolution axis symmetryand arranged with the silica substrate 51, the rotation motor (notshown), the powdered raw material's hopper 303 containing the powderedinner-layer's raw material 12 for forming the inner layer 56, theagitating screw 304, the measuring feeder 305, the carbon electrodes 212which are the heat source of the discharge-heat melting, the electricwirings 212 a, the high voltage electricity source unit 211, the cap213, and so forth. In addition, in the case of controlling theatmospheric gas, in similar to those in Step 3, the O₂ gas-supplyingcylinder 411, the inert gas-supplying cylinder 412, the gasmixture-supplying pipe 420, the dehumidifying equipment 430, thedew-point-temperature-measuring thermometer 440, and so forth may bearranged.

The inner layer 56 is formed as follows; firstly, the outer frame 101 isset at the prescribed rotation speed, and then high voltage is loadedgradually from the high voltage electricity source unit 211 and at thesame time the powdered inner-layer's raw material 12 for forming theinner layer 56 (high purity powdered silica) is spread gradually fromtop of the silica substrate 51 from the raw material's hopper 303. Atthis time, the electric discharge has been started between the carbonelectrodes 212 so that inside the silica substrate 51 is in thetemperature range of melting of the powdered silica (estimatedtemperature ranging from about 1800 to about 2000° C.); and with this,the spread powdered inner-layer's raw material 12 becomes to meltedsilica particles thereby attaching to the inner surface of the silicasubstrate 51. A mechanism is employed such that the carbon electrodes212 arranged in the upper opening site of the silica substrate 51, afeeding port of the powdered raw material, and the cap 213 may changetheir positions relative to the silica substrate 51 to a certain degree;and by changing these positions, the inner layer 56 can be formed on theentire inner surface of the silica substrate 51 with a uniformthickness.

In the formation of the inner layer 56, in similar to those in theformation of the silica substrate 51 in Step 3 as described above, theatmospheric gas during fusing and sintering by the discharge-heating canbe made under the condition of controlled humidity in the outer frame101 by charging the gas mixture having its dew-point temperature madebelow the prescribed value and comprised of an O₂ gas and an inert gaswhile ventilating the gases inside the outer frame 101.

Specifically, as shown in FIG. 9, an O₂ gas in the O₂ gas-supplyingcylinder 411 and an inert gas (for example, nitrogen or argon) in theinert gas-supplying cylinder 412 are mixed and charged from inside thesilica substrate 51 through the gas mixture-supplying pipe 420.Meanwhile, white hollow arrows shown by the code number 510 show theflow direction of the gas mixture. At this time, the gases in the outerframe 101 can be ventilated simultaneously, as mentioned above. Theventilation can be done, for example, by escaping the gases of theatmospheric inside the outer frame 101 to outside through a space in thecap 213. Meanwhile, white hollow arrows shown by the code number 520show the flow direction of the gas mixture with ventilation.

In the way as described above, the silica container 71′ of the presentinvention can be obtained; the silica container is cleaned as followingwhen it is necessary.

[Cleaning and Drying of the Silica Container]

For example, the silica container is etched on its surface by an aqueoussolution of hydrogen fluoride (HF) in the concentration range from about1 to about 10% with the time for 5 to 30 minutes, washed by pure water,and then dried in a clean air.

By executing the foregoing steps, the silica container 71 and 71′ of thepresent invention as mentioned above and shown in FIG. 1 and FIG. 2 canbe produced.

According to the method for producing a silica container of the presentinvention as mentioned above, the invention can be executed by mereadding of widely used equipment such as a dehumidifying equipment tothose of a conventional method without adding a special equipment and aprocess step; and thus a silica container containing extremely smallamounts of carbon and an OH group can be produced in a high dimensionalprecision, a high productivity, and a low cost.

EXAMPLE

Hereinafter, the present invention will be explained more specificallyby showing Examples and Comparative Examples of the present invention;but the present invention is not limited to them.

Example 1

According to the method for producing a silica container of the presentinvention as shown in FIG. 3, the silica container was produced asfollowing.

Firstly, as shown in FIG. 3 (1), the powdered substrate's raw material11 was prepared as following (Step 1).

Natural silica stones (100 kg) were prepared, heated in an airatmosphere at 1000° C. for 10 hours, poured into a pool of pure waterwhereby the stones were cooled quickly. After drying, the stones werecrushed by a crusher to make total weight 90 kg of the powdered silica(the powdered natural silica stone) having particle diameter in therange from 50 to 500 μm and silica purity (SiO₂) of 99.999% or higher byweight.

Then, as shown in FIG. 3 (2) and FIG. 6, the powdered substrate's rawmaterial 11 was fed to the inner wall 102 of the rotating outer frame101, which is made of graphite with a column-like shape and has theaspiration holes 103 formed in the inner wall 102; a shape of thepowdered substrate's raw material 11 was adjusted so as to obtainuniform thickness in accordance with the shape of the outer frame 101;in this way, the preliminarily molded silica substrate 41 was formed(Step 2).

Then, as shown in FIG. 3 (3), FIG. 7, and FIG. 8, the silica substrate51 was formed by the discharge-heat melting method under aspiration(Step 3).

Specifically, at first an O₂ gas in the O₂ gas-supplying cylinder 411and a nitrogen gas in the inert gas-supplying cylinder 412 were mixed(60% by volume of N₂ and 40% by volume of O₂) and then dehumidified bythe dehumidifying equipment 430 to the dew-point temperature of 10° C.(measured by the dew-point-temperature-measuring thermometer 440); theresulting gas mixture was charged from inside the preliminarily moldedsilica substrate 41 through the gas mixture-supplying pipe 420 (whitehollow arrow 510). On the other hand, gases in the outer frame 101 wereventilated (white hollow arrow 520). Under this condition, thepreliminarily molded silica substrate 41 was aspirated for degassingfrom the outer peripheral side of the preliminarily molded silicasubstrate 41 (degree of aspiration was about 10⁴ Pa) through theaspiration holes 103 formed in the outer frame 101, at the same time thepreliminarily molded silica substrate 41 was heated from the insidethereof at high temperature by the discharge-heat melting method withthe carbon electrodes 212; with this, the outer peripheral part of thepreliminarily molded silica substrate 41 was made to a sintered body andthe inner part of the preliminarily molded silica substrate 41 was madeto a fused glass body, thereby forming the silica substrate 51, whichwas made to the silica container 71.

The silica container 71 thus obtained was washed with 3% by weight of anaqueous solution of hydrogen fluoride (HE) for 3 minutes, rinsed withpure water, and then dried.

Example 2

Basically the same procedures as Example 1 were followed to produce thesilica container 71, except for the following changes. The atmosphericgas at the time when the preliminarily molded silica substrate 41 wasmelted and sintered in Step 3 was made to a gas mixture including 80% byvolume of N₂ and 20% by volume of O₂, the dew-point temperature wascontrolled at 7° C., and the amount of water vapor contained therein wasmade small.

Example 3

Basically the same procedures as Example 1 were followed to produce thesilica container 71, except for the following changes. The atmosphericgas at the time when the preliminarily molded silica substrate 41 wasmelted and sintered in Step 3 was made to a gas mixture including 90% byvolume of N₂ and 10% by volume of O₂, the dew-point temperature wascontrolled at 5° C., and the amount of water vapor contained therein wasmade small. At the same time, degree of vacuum by aspiration from theouter peripheral side was made higher as compared with Example 1 bycontrolled at 10³ Pa or lower.

Example 4

Basically the same procedures as Example 3 were followed to produce thesilica container 71, except for the following changes. The atmosphericgas at the time when the preliminarily molded silica substrate 41 wasmelted and sintered in Step 3 was made to a gas mixture including 95% byvolume of N₂ and 5% by volume of O₂, and the dew-point temperature wascontrolled at 5° C.

Example 5

According to the method for producing a silica container of the presentinvention as shown in FIG. 4, the silica container 71′ was produced asfollowing.

Firstly, Step 1 and Step 2 shown in FIGS. 4 (1) and (2) were followed ina similar manner to those of Step 1 and Step 2 of Example 1.

Then, procedures in Step 3 shown in FIG. 4 (3), FIG. 7, and FIG. 8 werefollowed in a similar manner to those of Step 3 in Example 1 except forthe following changes. The atmospheric gas to be charged was made to agas mixture including 90% by volume of N₂ and 10% by volume of O₂, thedew-point temperature was controlled at 5° C., and the amount of watervapor contained therein was made small.

Then, as shown in FIG. 4 (4) and FIG. 9 and described below, the innerlayer 56 formed of a transparent silica glass was formed on an innerwall surface of the silica substrate 51 formed in Step 1 to Step 3thereby making the silica container 71′ (Step 4).

Firstly, a high purity powdered synthetic cristobalite (particlediameter in the range from 100 to 300 and silica purity of 99.9999% orhigher by weight) was prepared as the powdered inner-layer's rawmaterial 12. To this powdered inner-layer's raw material 12 was added3×10¹⁷ molecules/g of an H₂ gas. Then, the powdered inner-layer's rawmaterial 12 was spread from inside the silica substrate 51 underatmosphere of the gas mixture comprised of 90% by volume of N₂ and 10%by volume of O₂ with the dew-point temperature controlled at 5° C. whileheating from inside thereof by the discharge-heat melting method therebyforming the inner layer 56 (thickness of 3 mm) formed of a transparentsilica glass on the inner surface of the silica substrate 51; with this,the silica container 71′ was made (Step 4).

Similar to Example 1, the silica container 71′ thus obtained was washedwith 3% by weight of an aqueous solution of hydrogen fluoride (HF) for 3minutes, rinsed with pure water, and then dried.

Example 6

The silica container 71′ was produced in a manner similar to those ofExample 5, except that aluminum was added to the powdered substrate'sraw material 11 so as to be contained therein with the concentration of60 ppm by weight and the powdered inner-layer's raw material 12 waschanged to a powdered synthetic cristobalite added with 3×10¹⁸molecules/g of H₂.

Example 7

The silica container 71 was produced in a manner similar to those ofExample 1 and Example 2, except for the following changes. Theatmospheric gas at the time when the preliminarily molded silicasubstrate 41 was melted and sintered in Step 3 was made to a gas mixtureincluding 90% by volume of N₂ and 10% by volume of O₂, the dew-pointtemperature was controlled at −15° C., and the amount of water vaporcontained therein was made small.

Example 8

The silica container 71 was produced in a manner similar to those ofExample 7, except for the following changes. While the atmospheric gasat the time when the preliminarily molded silica substrate 41 was meltedand sintered in Step 3 was still kept as a gas mixture including 90% byvolume of N₂ and 10% by volume of O₂, the dew-point temperature wascontrolled at −20° C., and the amount of water vapor contained thereinwas made small.

Example 9

The silica container 71′ was produced in a manner similar to those ofExample 5, except for the following changes. Aluminum was added to thepowdered substrate's raw material 11 so as to be contained therein withthe concentration of 60 ppm by weight; and while the atmospheric gas atthe time when the preliminarily molded silica substrate 41 was meltedand sintered in Step 3 was still kept as a gas mixture including 90% byvolume of N₂ and 10% by volume of O₂, the dew-point temperature wascontrolled at −15° C.

Example 10

The silica container 71′ was produced in a manner similar to those ofExample 6, except for the following changes. While the atmospheric gasat the time when the preliminarily molded silica substrate 41 was meltedand sintered in Step 3 was still kept as a gas mixture including 90% byvolume of N₂ and 10% by volume of O₂, the dew-point temperature wascontrolled at −20° C.

Comparative Example 1

According to mostly a conventional method, a silica container (a silicacrucible) was prepared. Namely, a part corresponding to the silicasubstrate of the silica container of the present invention was formed byalso using a high purity powdered raw material, under an air atmospherewithout humidity control by the discharge-heat melting method.

Firstly, a high purity powdered natural quartz having silica purity of99.9999% or higher by weight (particle diameter in the range from 100 to300 μm) was prepared as the powdered raw material corresponding to thepowdered substrate's raw material.

By using this powdered raw material under an air atmosphere withoutspecific humidity control, this high purity powdered natural quartz wasfed to a rotating frame made of graphite by utilizing a centrifugalforce to form a powdered quartz layer in the rotating frame; the layerwas then melted by discharge-heating with the carbon electrodes to formthe silica substrate (corresponding to the silica substrate 51 of thepresent invention shown in FIG. 1). Time of this procedure was 60minutes, and temperature near the inner surface of the silica substratewas estimated to be about 2000° C.

Comparative Example 2

In a similar manner to that of Comparative Example 1, firstly, thesilica substrate (corresponding to the silica substrate 51 of thepresent invention as shown in FIG. 2) was formed.

Then, the same powdered synthetic cristobalite as in Examples 5, 6, 9,and 10 was prepared as the powdered raw material corresponding to thepowdered inner-layer's raw material; then this high purity powderedsynthetic cristobalite was spread from the hopper onto an inner surfaceof the silica substrate and was melted by the discharge-heating methodwith the carbon electrodes under an air atmosphere without specifichumidity control to form the inner layer (corresponding to the innerlayer 56 in the silica container 71′ of the present invention, as shownin FIG. 2).

EVALUATION METHODS IN EXAMPLES AND COMPARATIVE EXAMPLES

Measurements of physical properties and property evaluation of thepowdered raw material and gases used and the silica container producedin each Example and Comparative Example were carried out as shown below.

Method for Measuring Particle Diameter of Each Powdered Raw Material

Two-dimensional shape observation and area measurement of each powderedraw material were carried out with an optical microscope or an electronmicroscope. Then the diameter was obtained by calculation of theobtained area value with the assumption that shape of the particle is atrue circle. This technique was repeated statistically to obtain therange of particle diameter (99% or more by weight of particles areincluded in this range).

Measurement of the Dew-Point Temperature

Measurement was done with a dew-point-temperature-measuring thermometer.

Meanwhile, the measurement in each Example was done by thedew-point-temperature-measuring thermometer 440 arranged in the gasmixture-supplying pipe 420, as mentioned above.

Analysis of the Impure Metal Element Concentration

When the impure metal element concentration is relatively low (i.e., theglass is of high purity), ICP-AES (Inductively Coupled Plasma-AtomicEmission Spectroscopy) or ICP-MS (Inductively Coupled Plasma-MassSpectroscopy) was used, and when the impure metal element concentrationis relatively high (i.e., the glass is of low purity), AAS (AtomicAbsorption Spectroscopy) was used.

Thickness Measurement of Each Layer

The container cross section at the half part of total height of sidewall of the silica container was measured by a scale to obtain thicknessof the silica substrate and the inner layer.

Measurement of Concentration of an OH Group

The measurement was done with an infrared absorption spectroscopy.Conversion to concentration of an OH group was done according to thefollowing literature:

-   Dodd, D. M. and Fraser, D. B., (1966), “Optical determination of OH    in fused silica”, Journal of Applied Physics, vol. 37, p. 3911.    Method for Measurement of Amount of Gas Molecules Released from Each    of the Silica Substrates and the Inner Layers

Each measurement sample of granules having the particle diameter in therange from 100 μm to 1 mm was prepared from the inner part of the silicasubstrate not containing gaseous bubbles (a colorless and transparentlayer part, or in the case of Comparative Examples 1 an inner-most parthaving relatively small amount of gaseous bubbles) and from the innerlayer of each of the silica containers in Examples and ComparativeExamples, and then the sample thus obtained was arranged in a vacuumchamber; amounts and kinds of the released gases at 1000° C. undervacuum was then measured with a mass analyzer.

It was assumed that all amounts of an H₂O gas, a CO gas, and a CO₂ gaswere released; and the amount of each gas was expressed by numbers ofthe released molecules per unit mass (molecules/g). Details of themeasurement were followed according to the following literature:

Nasu, S., et al., (1990), “Gas release of various kinds of vitreoussilica”, Journal of Illuminating Engineering Institute of Japan, vol.74, No. 9, pp 595 to 600.

Analysis of the Amount of Contained Carbon Element (C)

With regard to each layer of the silica containers in Examples andComparative Examples, a granular sample having the diameter controlledin the range from 100 μm to 1 mm was prepared for measurement. Thisgranular sample for measurement was prepared in a sample chamber andburned with a high-frequency induction heating under an oxygengas-containing atmosphere; and amounts of carbon monoxide (CO) andcarbon dioxide (CO₂) produced by reaction of the sample with an oxygengas were quantitatively analyzed by an infrared absorption method toanalyze the amount of carbon element contained therein.

Viscosity

A material with the size of about 10×10 cm was cut out from each silicacontainer, rinsed, and then kept in a clean electric furnace under anair atmosphere at 1150° C. for 3 hours. Then, the temperature wasdescended to 900° C. at the descendent rate of 10° C./hour; thereafter,the electric switch was turned off to allow the temperature be descendeddown to room temperature naturally in the electric furnace. With thisheat treatment, the materials cut out from respective silica containerswere made equal in their thermal histories. Then, each measurementsample having the size of 3×10 mm with 100 mm length and mirror-polishedon the entire surface was prepared from the inside part of the silicasubstrate of the material not having gaseous bubbles (a colorless andtransparent layer part, or in Comparative Examples 1 and 2 an inner-mostpart having relatively small amount of gaseous bubbles). Then, viscosityη at 1400° C. was measured by a beam bending method. Details werefollowed in the literature shown below.

Yoshikazu Kikuchi, et al., (1997), “OH Content Dependence of Viscosityof Vitreous Silica”, Journal of the Ceramics Society of Japan, Vol. 105,No. 8, pp. 645-649.

Evaluation of Continuous Pulling Up of a Single Crystal Silicon(Multipulling)

A metal polysilicon with purity of 99.999999% by weight was fed into aproduced silica container; thereafter, the temperature was raised toform a silicon melt, and then pulling up of a single crystal silicon wasrepeated for three times (multipulling). The evaluation was made as thesuccess rate of single crystal growth. The pulling up conditions were:atmosphere of 100% of an argon gas (Ar) with the pressure inside the CZequipment being 10³ Pa, 1 mm/minute of the pulling up rate, rotationnumbers of 10 rpm, and size of the single crystal silicon with 150 mm indiameter and 150 mm in length. Operation time for one batch was set atabout 12 hours. Classification of evaluation based on the success rateof single crystal growth for repetition of three times was made asfollowing:

three times: good two times: fair one time: poor

Evaluation of the Thermal Distortion Resistance of a Silica Container

In evaluation of the multipulling of a single crystal silicon asmentioned above, amount of collapse of the side wall upper part of asilica container toward inside thereof after the third pulling up wasevaluated as following:

amount of collapse toward inside was less than 1 good cm: amount ofcollapse toward inside was 1 cm or more fair and less than 2 cm: amountof collapse toward inside was 2 cm or more: poor

Evaluation of Voids and Pinholes

In the foregoing multipulling of the single crystal silicon, 10 sheetsof silicon wafer having the size of 150 mm diameter and 200 μm thicknessand polished on the both sides were prepared from an arbitrary portionof the first single crystal silicon after multipulling of each singlecrystal silicon. Then, voids and pinholes present on the both sides ofeach silicon wafer were counted; average void numbers and pinholenumbers per unit area (m²) were obtained by a statistic numericaltreatment.

average number of voids and pinhole is less than good 1/m² averagenumber of voids and pinhole is in the fair range from 1 to 2/m² averagenumber of voids and pinhole is more than poor 3/m²

Evaluation of (Relative) Production Cost of a Silica Container

The production cost of the silica container was evaluated. Inparticular, costs of silica raw materials, a melting energy, and soforth were summed up for the relative evaluation.

low cost (less than 90% relative to cost of a good conventional method):moderate cost (90 to 110% relative to cost of a fair conventionalmethod): high cost (higher than 110% relative to cost of a poorconventional method):

Production conditions, measured physical properties, and evaluationresults of each silica container produced in Examples 1 to 10 andComparative Examples 1 to 2 are summarized in the following Tables 1 to6 and Table 7. Analysis data of impurity in the inner layer are shown inTable 7.

TABLE 1 Example No. Example 1 Example 2 Powdered substrate's rawmaterial Powdered natural silica Powdered natural silica Particlediameter: 50 to Particle diameter: 50 to 500 μm 500 μm Silica purity:99.999% Silica purity: 99.999% by weight by weight Powderedinner-layer's raw material No No Amount of H₂ released from powderedinner- — — layer's raw material (molecules/g) Preliminary molding methodof silica Rotation molding within Rotation molding within substrategraphite frame graphite frame Melting-sintering method of silica Arcdischarge melting Arc discharge melting substrate under aspiration underaspiration Atmospheric gas during silica substrate N₂: 60% by volume,N₂: 80% by volume, formation O₂: 40% by volume O₂: 20% by volumeDew-point temperature: Dew-point temperature: 10° C. 7° C. Meltingmethod of inner layer — — Atmospheric gas during inner layer melting — —Physical Color tone White opaque to White opaque to properties ofcolorless transparent colorless transparent silica Outer diameter,height, Outer diameter 450/ Outer diameter 450/ substrate thickness (mm)height 450/thickness 13 height 450/thickness 13 OH Concentration 30 15(ppm by weight) Carbon Concentration <10 <10 (ppm by weight) Amount ofCO release 2 × 10¹⁷ 1 × 10¹⁷ (molecules/g) Amount of CO₂ release 1 ×10¹⁷ <1 × 10¹⁷  (molecules/g) Amount of H₂O release 3 × 10¹⁷ 2 × 10¹⁷(molecules/g) Al Concentration 5 5 (ppm by weight) log(η/Pa · s) ofcolorless and 10.4 10.4 transparent layer at 1400° C.(η: viscosity(Pa ·s)) Physical Color tone — — properties of Thickness (mm) — — inner layerOH Concentration — — (ppm by weight) Carbon Concentration — — (ppm byweight) Evaluation Silicon single crystal Fair Fair multipulling Thermaldistortion Fair Fair resistance Void/pinhole of silicon Fair Good singlecrystal Cost Good Good

TABLE 2 Example No. Example 3 Example 4 Powdered substrate's rawmaterial Powdered natural silica Powdered natural silica Particlediameter: 50 to Particle diameter: 50 to 500 μm 500 μm Silica purity:99.999% Silica purity: 99.999% by weight by weight Powderedinner-layer's raw material No No Amount of H₂ released from powderedinner- — — layer's raw material (molecules/g) Preliminary molding methodof silica Rotation molding within Rotation molding within substrategraphite frame graphite frame Melting-sintering method of silica Arcdischarge melting Arc discharge melting substrate under strongaspiration under strong aspiration Atmospheric gas during silicasubstrate N₂: 90% by volume, N₂: 95% by volume, formation O₂: 10% byvolume O₂: 5% by volume Dew-point temperature: Dew-point temperature: 5°C. 5° C. Melting method of inner layer — — Atmospheric gas during innerlayer melting — — Physical Color tone White opaque to White opaque toproperties of colorless transparent colorless transparent silica Outerdiameter, height, Outer diameter 450/ Outer diameter 450/ substratethickness (mm) height 450/thickness 13 height 450/thickness 13 OHConcentration 10 15 (ppm by weight) Carbon Concentration 10 20 (ppm byweight) Amount of CO release <1 × 10¹⁷ <1 × 10¹⁷ (molecules/g) Amount ofCO₂ release <1 × 10¹⁷ <1 × 10¹⁷ (molecules/g) Amount of H₂O release  1 ×10¹⁷  1 × 10¹⁷ (molecules/g) Al Concentration 5 5 (ppm by weight)log(η/Pa · s) of colorless and 10.5 10.5 transparent layer at 1400°C.(η: viscosity(Pa · s)) Physical Color tone — — properties of Thickness(mm) — — inner layer OH Concentration — — (ppm by weight) CarbonConcentration — — (ppm by weight) Evaluation Silicon single crystal GoodFair multipulling Thermal distortion Fair Fair resistance Void/pinholeof silicon Good Good single crystal Cost Good Good

TABLE 3 Example No. Example 5 Example 6 Powdered substrate's rawmaterial Powdered natural silica Powdered natural silica Particlediameter: 50 to Particle diameter: 50 to 500 μm 500 μm Silica purity:99.999% Silica purity: 99.999% by weight by weight Powderedinner-layer's raw material Powdered synthetic Powdered syntheticcristobalite cristobalite Particle diameter: 100 Particle diameter: 100to 300 μm to 300 μm Silica purity: 99.9999% Silica purity: 99.9999% byweight by weight Amount of H₂ released from powdered inner- 3 × 10¹⁷ 3 ×10¹⁸ layer's raw material (molecules/g) Preliminary molding method ofsilica Rotation molding within Rotation molding within substrategraphite frame graphite frame Melting-sintering method of silica Arcdischarge melting Arc discharge melting substrate under aspiration underaspiration Atmospheric gas during silica substrate N₂: 90% by volume,N₂: 90% by volume, formation O₂: 10% by volume O₂: 10% by volumeDew-point temperature: Dew-point temperature: 5° C. 5° C. Melting methodof inner layer Normal pressure arc Normal pressure arc discharge withspreading discharge with spreading of powdered raw material of powderedraw material Atmospheric gas during inner layer melting N₂: 90% byvolume, N₂: 90% by volume, O₂: 10% by volume O₂: 10% by volume Dew-pointtemperature: Dew-point temperature: 5° C. 5° C. Physical Color toneWhite opaque to White opaque to properties colorless transparentcolorless transparent of silica Outer diameter, height, Outer diameter450/ Outer diameter 450/ substrate thickness (mm) height 450/thickness13 height 450/thickness 13 OH Concentration 10 10 (ppm by weight) CarbonConcentration 10 10 (ppm by weight) Amount of CO release 1 × 10¹⁷ 1 ×10¹⁷ (molecules/g) Amount of CO₂ release 1 × 10¹⁷ 1 × 10¹⁷ (molecules/g)Amount of H₂O release 1 × 10¹⁷ <1 × 10¹⁷  (molecules/g) Al Concentration5 60 (ppm by weight) log(η/Pa · s) of colorless and 10.6 10.7transparent layer at 1400° C.(η: viscosity(Pa · s)) Physical Color toneColorless and Colorless and properties of transparent transparent innerlayer Thickness (mm) 3 3 OH Concentration 15 20 (ppm by weight) CarbonConcentration <10 <10 (ppm by weight) Evaluation Silicon single crystalGood Good multipulling Thermal distortion Good Good resistanceVoid/pinhole of silicon Good Good single crystal Cost Fair Fair

TABLE 4 Example No. Example 7 Example 8 Powdered substrate's rawmaterial Powdered natural silica Powdered natural silica Particlediameter: 50 to Particle diameter: 50 to 500 μm 500 μm Silica purity:99.999% Silica purity: 99.999% by weight by weight Powderedinner-layer's raw material No No Amount of H₂ released from powderedinner- — — layer's raw material (molecules/g) Preliminary molding methodof silica Rotation molding within Rotation molding within substrategraphite frame graphite frame Melting-sintering method of silica Arcdischarge melting Arc discharge melting substrate under aspiration underaspiration Atmospheric gas during silica substrate N₂: 90% by volume,N₂: 90% by volume, formation O₂: 10% by volume O₂: 10% by volumeDew-point Dew-point temperature: −15° C. temperature: −20° C. Meltingmethod of inner layer — — Atmospheric gas during inner layer melting — —Physical Color tone White opaque to White opaque to properties colorlesstransparent colorless transparent of silica Diameter, height, thicknessOuter diameter 450/ Outer diameter 450/ substrate (mm) height450/thickness 13 height 450/thickness 13 OH Concentration 8 5 (ppm byweight) Carbon Concentration 10 10 (ppm by weight) Amount of CO release<1 × 10¹⁷ <1 × 10¹⁷ (molecules/g) Amount of CO₂ release <1 × 10¹⁷ <1 ×10¹⁷ (molecules/g) Amount of H₂O release <1 × 10¹⁷ <1 × 10¹⁷(molecules/g) Al Concentration 5 5 (ppm by weight) log(η/Pa · s) ofcolorless and 10.6 10.6 transparent layer at 1400° C.(η: viscosity(Pa ·s)) Physical Color tone — — properties of Thickness (mm) — — inner layerOH Concentration — — (ppm by weight) Carbon Concentration — — (ppm byweight) Evaluation Silicon single crystal Good Good multipulling Thermaldistortion Good Good resistance Void/pinhole of silicon Good Good singlecrystal Cost Good Good

TABLE 5 Example No. Example 9 Example 10 Powdered substrate's rawmaterial Powdered natural silica Powdered natural silica Particlediameter: 50 to Particle diameter: 50 to 500 μm 500 μm Silica purity:99.999% Silica purity: 99.999% by weight by weight Powderedinner-layer's raw material Powdered synthetic Powdered syntheticcristobalite cristobalite Particle diameter: 100 Particle diameter: 100to 300 μm to 300 μm Silica purity: 99.9999% Silica purity: 99.9999% byweight by weight Amount of H₂ released from powdered inner-  3 × 10¹⁷  3× 10¹⁸ layer's raw material (molecules/g) Preliminary molding method ofsilica Rotation molding within Rotation molding within substrategraphite frame graphite frame Melting-sintering method of silica Arcdischarge melting Arc discharge melting substrate under aspiration underaspiration Atmospheric gas during silica substrate N₂: 90% by volume,N₂: 90% by volume, formation O₂: 10% by volume O₂: 10% by volumeDew-point temperature: Dew-point temperature: −15° C. −20° C. Meltingmethod of inner layer Normal pressure arc Normal pressure arc dischargewith spreading discharge with spreading of powdered raw material ofpowdered raw material Atmospheric gas during inner layer melting N₂: 90%by volume, N₂: 90% by volume, O₂: 10% by volume O₂: 10% by volumeDew-point temperature: Dew-point temperature: 5° C. 5° C. Physical Colortone White opaque to White opaque to properties colorless transparentcolorless transparent of silica Outer diameter, height, Outer diameter450/ Outer diameter 450/ substrate thickness (mm) height 450/thickness13 height 450/thickness 13 OH Concentration 8 5 (ppm by weight) CarbonConcentration 10 10 (ppm by weight) Amount of CO release <1 × 10¹⁷ <1 ×10¹⁷ (molecules/g) Amount of CO₂ release <1 × 10¹⁷ <1 × 10¹⁷(molecules/g) Amount of H₂O release <1 × 10¹⁷ <1 × 10¹⁷ (molecules/g) AlConcentration 60 60 (ppm by weight) log(η/Pa · s) of colorless and 10.710.8 transparent layer at 1400° C.(η: viscosity(Pa · s)) Physical Colortone Colorless and Colorless and properties of transparent transparentinner layer Thickness (mm) 3 3 OH Concentration 15 20 (ppm by weight)Carbon Concentration <10 <10 (ppm by weight) Evaluation Silicon singlecrystal Good Good multipulling Thermal distortion Good Good resistanceVoid/pinhole of silicon Good Good single crystal Cost Fair Fair

TABLE 6 Example No. Comparative Example 1 Comparative Example 2 Powderedsubstrate's raw material Powdered natural silica Powdered natural silicaParticle diameter: 100 to 300 Particle diameter: 100 to 300 μm μm Silicapurity: 99.9999% by Silica purity: 99.9999% by weight weight Powderedinner-layer's raw material No Powdered synthetic cristobalite Particlediameter: 100 to 300 μm Silica purity: 99.9999% by weight Amount of H₂released from powdered inner- — <1 × 10¹⁶  layer's raw material(molecules/g) Preliminary molding method of silica Rotation moldingwithin Rotation molding within substrate graphite frame graphite frameMelting-sintering method of silica Normal pressure arc discharge Normalpressure arc discharge substrate melting melting Atmospheric gas duringsilica substrate Air, without dehumidification Air, withoutdehumidification formation (Measured dew-point (Measured dew-pointtemperature: 15° C.) temperature: 17° C.) Melting method of inner layer— Normal pressure arc discharge with spreading of powdered raw materialAtmospheric gas during inner layer melting — Air, withoutdehumidification (Measured dew-point temperature: 17° C.) Physical Colortone White opaque White opaque properties of silica Outer diameter,height, Outer diameter 450/height Outer diameter 450/height substratethickness (mm) 450/thickness 13 450/thickness 13 OH Concentration 100 90(ppm by weight) Carbon Concentration 60 50 (ppm by weight) Amount of COrelease 4 × 10¹⁷ 3 × 10¹⁷ (molecules/g) Amount of CO₂ release 2 × 10¹⁷ 2× 10¹⁷ (molecules/g) Amount of H₂O release 4 × 10¹⁷ 5 × 10¹⁷(molecules/g) Al Concentration 5 5 (ppm by weight) log(η/Pa · s) ofcolorless and 10.2 10.2 transparent layer at 1400° C.(η: viscosity(Pa ·s)) Physical Color tone — Colorless and transparent properties ofThickness (mm) — 3 inner layer OH Concentration — 130 (ppm by weight)Carbon Concentration — 30 (ppm by weight) Evaluation Silicon singlecrystal Poor Poor multipulling Thermal distortion Fair Fair resistanceVoid/pinhole of silicon Poor Fair single crystal Cost Fair Poor

TABLE 7 (Unit: ppb by weight) Comparative Element Example 5 Example 6Example 9 Example 10 Example 2 Li 10 8 6 6 9 Na 42 40 41 40 40 K 23 2122 23 20 Ti 5 6 6 5 5 V 2 3 2 2 2 Cr 10 8 9 9 8 Fe 25 20 23 22 25 Co 2 22 2 1 Ni 10 5 10 10 10 Cu 6 6 6 5 7 Zn 3 2 2 3 3 Mo 2 2 2 1 1 W 1 1 1 11

As can be seen in Tables 1 to 5, in Examples 1 to 10 executed inaccordance with the method for producing a silica container of thepresent invention, silica containers giving the results in pulling up ofa single crystal no way inferior to a conventional silica container ofComparative Examples 1 and 2 could be obtained, even if silicacontainers produced in a low cost and a high productivity were used InExamples 1 to 10, concentrations of carbon and an OH group contained inthe silica container could be made lower as compared with ComparativeExamples 1 and 2. Especially in Examples 5, 6, 9, and 10, in which theinner layer 56 was formed, amount of impure metal elements in the innerlayer was in the same level as Comparative Example 2, as can be seen inTable 7; and thus it can be seen that a single crystal with asufficiently high purity could be pulled up.

Accordingly, in Examples 1 to 10, multipulling of a single crystalsilicon could be done with a high success rate; and thus formation ofvoids and pinholes could be made in the same level or lower as comparedwith Comparative Examples 1 and 2.

The thermal distortion resistance could be improved.

It must be noted here that the present invention is not limited to theembodiments as described above. The foregoing embodiments are mereexamples; any form having substantially the same composition as thetechnical concept described in claims of the present invention andshowing similar effects is included in the technical scope of thepresent invention.

1-14. (canceled)
 15. A method for producing a silica container comprisedof at least a silica as its main component and arranged with a silicasubstrate having a rotational symmetry, wherein the method comprises: astep of preparing a powdered substrate's raw material comprised ofsilica particles for forming the silica substrate, a step of forming apreliminarily molded silica substrate, wherein the powdered substrate'sraw material is fed to an inner wall of an outer frame having arotational symmetry and aspiration holes arranged splittingly in theinner wall while rotating the outer frame thereby preliminarily moldingthe powdered substrate's raw material to an intended shape in accordancewith the inner wall of the outer frame, and a step of forming the silicasubstrate, wherein the preliminarily molded silica substrate is degassedby aspiration from an outer peripheral side through the aspiration holesformed in the outer frame with controlling a humidity inside the outerframe by ventilating gases present in the outer frame with charging frominside the preliminarily molded silica substrate a gas mixture comprisedof an O₂ gas and an inert gas and made below a prescribed dew-pointtemperature by dehumidification, and at the same time heated from insidethe preliminarily molded silica substrate by a discharge-heat meltingmethod with carbon electrodes, thereby making an outer peripheral partof the preliminarily molded silica substrate to a sintered body while aninner peripheral part of the preliminarily molded silica substrate to afused glass body.
 16. A method for producing a silica container, whereinthe method for producing a silica container according to claim 15further includes, after the step of forming the silica substrate by thedischarge-heat melting method, a step of forming an inner layer formedof a transparent silica glass on an inner surface of the silicasubstrate, by spreading from inside the silica substrate a powderedinner-layer's raw material, comprised of silica particles and having ahigher silica purity than the powdered substrate's raw material, withheating from inside the silica substrate by a discharge-heat meltingmethod.
 17. The method for producing a silica container according toclaim 16, wherein the powdered inner-layer's raw material is made to theone that releases H₂ the amount of which is in the range from 1×10¹⁶ to1×10¹⁹ molecules/g at 1000° C. under vacuum.
 18. The method forproducing a silica container according to claim 16, wherein, in thepowdered inner-layer's raw material, each element concentration of Li,Na, and K is made 60 or less ppb by weight, and each elementconcentration of Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo, and W is made 30 orless ppb by weight.
 19. The method for producing a silica containeraccording to claim 17, wherein, in the powdered inner-layer's rawmaterial, each element concentration of Li, Na, and K is made 60 or lessppb by weight, and each element concentration of Ti, V, Cr, Fe, Co, Ni,Cu, Zn, Mo, and W is made 30 or less ppb by weight.
 20. The method forproducing a silica container according to claim 15, wherein the ratio ofan O₂ gas contained in the gas mixture is in the range from 1 to 40% byvolume.
 21. The method for producing a silica container according toclaim 16, wherein the ratio of an O₂ gas contained in the gas mixture isin the range from 1 to 40% by volume.
 22. The method for producing asilica container according to claim 15, wherein the dehumidification isdone such that a dew-point temperature of the gas mixture may become 10°C. or lower.
 23. The method for producing a silica container accordingto claim 16, wherein the dehumidification is done such that a dew-pointtemperature of the gas mixture may become 10° C. or lower.
 24. Themethod for producing a silica container according to claim 15, whereinthe powdered substrate's raw material is made to contain aluminum in theconcentration rage from 10 to 500 ppm by weight.
 25. The method forproducing a silica container according to claim 16, wherein the powderedsubstrate's raw material is made to contain aluminum in theconcentration rage from 10 to 500 ppm by weight.
 26. A silica containercomprised of at least a silica as its main component and arranged with asilica substrate having a rotational symmetry, wherein the silicasubstrate contains carbon in the concentration of 30 or less ppm byweight and an OH group in the concentration of 30 or less ppm by weightand has a white and opaque layer part containing gaseous bubbles in itsouter peripheral part and a colorless and transparent layer partcomprised of a silica glass not substantially containing gaseous bubblesin its inner peripheral part.
 27. The silica container according toclaim 26, wherein amounts of released gas molecules are 2×10¹⁷ or lessmolecules/g for a CO molecule and 1×10¹⁷ or less molecules/g for a CO₂molecule when the colorless and transparent layer part of the silicasubstrate is heated at 1000° C. under vacuum.
 28. The silica containeraccording to claim 26, wherein amount of a released H₂O molecule is3×10¹⁷ or less molecules/g when the colorless and transparent layer partof the silica substrate is heated at 1000° C. under vacuum.
 29. Thesilica container according to claim 26, wherein viscosity of thecolorless and transparent layer part of the silica substrate at 1400° C.is 10¹⁰ ⁴ Pa·s or higher.
 30. The silica container according to claim26, wherein the silica substrate contains aluminum in the concentrationrange from 10 to 500 ppm by weight.
 31. The silica container accordingto claim 26, wherein an inner layer formed of a transparent silica glasshaving a higher silica purity than the silica substrate is arranged onan inner surface of the silica substrate.
 32. The silica containeraccording to claim 31, wherein the inner layer contains carbon with theconcentration of 30 or less ppm by weight, an OH group with theconcentration of 30 or less ppm by weight, an element of Li, Na, and Kwith each element concentration of 60 or less ppb by weight, and anelement of Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo, and W with each elementconcentration of 30 or less ppb by weight.